Dissociative adsorption of alkanes on Ni(100): Comparison with molecular beam results

Publisher Website
Google Scholar
Abstract
The dissociative adsorption of ethane, propane, and n‐butane on Ni(100) has been investigated at pressures of 0.1–0.001 Torr and temperatures between 350 and 500 K. Activation energies for dissociation are found to be 9.5 kcal/mol for ethane, 3.8 kcal/mol for propane, and 3.1 kcal/mol for n‐butane. Dissociative sticking probabilities increase with increasing carbon chain length. Comparison of the present results with the results of a recent molecular beam study of alkane dissociation on Ni(100) show very poor agreement. The sticking probabilities measured here are all orders of magnitude higher than those predicted from the molecular beam study. The discrepancy is attributed to the behavior of alkane molecules with very low normal kinetic energies, which have sticking probabilities below the limits of detection of the beam experiment. C2 H6 and C2 D6 have identical dissociative sticking probabilities indicating that quantum tunneling of hydrogen is not an important step in ethane dissociation. Dissociation is proposed to occur via a trapped molecular precursor. The measured alkane dissociation rates compare favorably with hydrogenolysis and steam reforming rates.